This invention relates to a process for regulating the heating of a thermoplastic band used in a station for thermoforming upwardly open containers, of the type in which the thermoplastic band is firstly heated section by section over the major part of its width except for thin marginal zones adapted to be gripped by a step-by-step advance device, up to a softening temperature rendering it suitable for thermoforming, then said band is cooled so as to create therein a sort of meshed network of which the meshes are constituted by rigidified thermoplastic matter and each surround a zone of thermoplastic matter softened to the thermoforming temperature and corresponding to the extent and location of the upper opening of one of the forming chambers of a molding unit provided in the thermoforming station, and finally, the band thus cooled is advanced by one step into said thermoforming station, causing the softened zones to coincide with the openings of the forming chambers before proceeding with the thermoforming operation proper.
It has been proven that the softened zones of the thermoplastic band are not always at a temperature appropriate for thermoforming, with the result that the containers are torn during thermoforming or present wall zones of considerably unequal thicknesses.
It has already been proposed to measure the temperature of the thermoplastic band when it leaves the heating zone, but the temperature has proved not to be representative of the aptitude of the band for thermoforming.
It is an object of the present invention to overcome the drawbacks set forth hereinabove and to propose a process for regulating the heating of the thermoplastic band so that it is in all circumstances in the optimum state of thermoforming.